Method of making closed end ceramic fuel cell tubes

ABSTRACT

A method of manufacturing closed end ceramic fuel cell tubes with improved properties and higher manufacturing yield is disclosed. The method involves bonding an unfired cap to a hollow unfired tube to form a compound joint. The assembly is then fired to net shape without subsequent machining. The resultant closed end tube is superior in that it provides a leak-tight seal and its porosity is substantially identical to that of the tube wall. The higher manufacturing yield associated with the present method decreases overall fuel cell cost significantly.

GOVERNMENT CONTRACT

The Government of the United States of America has certain rights inthis invention pursuant to Contract No. DE-FC21-91MC28055 awarded by theU.S. Department of Energy.

FIELD OF THE INVENTION

The present invention relates to fuel cells, and more particularlyrelates to a method of making closed end ceramic tubes for solid oxidefuel cells and the like.

BACKGROUND INFORMATION

Fuel cells are among the most efficient of power generation devices. Onetype of solid oxide fuel cell (SOFC) generator has a projected 70percent net efficiency when used in an integrated SOFC-combustionturbine power system in which the turbine combustor is replaced by aSOFC.

Several different fuel cell designs are known. For example, one type ofsolid oxide fuel cell consists of an inner porous doped-lanthanummanganite tube having an open end and a closed end, which serves as thesupport structure for the individual cell, and is also the cathode orair electrode (AE) of the cell. A thin gas-tight yttria-stabilizedzirconia electrolyte covers the air electrode except for a relativelythin strip of an interconnection surface, which is a dense gas-tightlayer of doped-lanthanum chromite. This strip serves as the electriccontacting area to an adjacent cell or, alternatively, to a powercontact. A porous nickel-zirconia cermet layer, which is the anode orfuel electrode, covers the electrolyte, but not the interconnectionstrip. A typical closed end SOFC air electrode tube has a length ofabout 1.81 m, a diameter of about 2.2 cm and is used in a seal-less SOFCdesign.

Exemplary fuel cells are disclosed in U.S. Pat. No. 4,431,715 toIsenberg, U.S. Pat. No. 4,395,468 to Isenberg, U.S. Pat. No. 4,490,444to Isenberg, U.S. Pat. No. 4,562,124 to Ruka, U.S. Pat. No. 4,631,138 toRuka, U.S. Pat. No. 4,748,091 to Isenberg, U.S. Pat. No. 4,751,152 toZymboly, U.S. Pat. No. 4,791,035 to Reichner, U.S. Pat. No. 4,833,045 toPollack, et al., U.S. Pat. No. 4,874,678 to Reichner, U.S. Pat. No.4,876,163 to Reichner, U.S. Pat. No. 4,888,254 to Reichner, U.S. Pat.No. 5,103,871 to Misawa et al., U.S. Pat. No. 5,108,850 to Carlson etal., U.S. Pat. No. 5,112,544 to Misawa et al., U.S. Pat. No. 5,258,240to Di Croce et al., and U.S. Pat. No. 5,273,828 to Draper et al., eachof which is incorporated herein by reference.

The primary requirements of the closed end of the air electrode forcommercial applications are that it has properties that are similar tothose of the air electrode tube wall and can be rapidly fabricated,preferably in a high-volume manufacturing facility.

Different techniques have conventionally been used to form the closedend of the air electrode tube. One method is referred to as the pressedplug technique. This process involves forming a rod of air electrodematerial by extrusion, inserting the rod into a dried, green tube, andapplying a uniaxial load. This technique is problematic in that the loadapplied to the plug material must be sufficient to achieve an adequatebond between the plug and the tube material, but must not be so great asto break the tube. This method also requires controlled drying in orderto minimize the possibility of debonding of the plug from the walland/or cracking of the plug. Plugs made by this method also requiremachining of the sintered plugged end. The most common problem found intubes made with this technique is poor bonding at the plug/wallinterface. Furthermore, this technique cannot be used to produce closedend ribbed tubular air electrodes, which are being considered for theirpotential performance enhancement.

An alternate method that has been used to manufacture air electrodetubes is referred to as the cast plug technique. This method involvesinserting a cellulose preform into a dried, green tube in order todefine the plug internal radius. An air electrode slurry comprising awater-based suspension of AE particles is deposited or cast onto thepreform. Precise control of the plug slurry rheology is required toensure reproducibility. This assembly is then dried slowly in acontrolled humidity and temperature chamber to prevent debonding of theplug from the tube wall or the formation of cracks in the plug. Once theair electrode is dry, it is sintered to the desired density and theplugged end is machined or ground to the proper hemispherical radius andplug thickness. The most common problems found in tubes made with thistechnique are a large difference in porosity between the tube wall andthe plug, and poor bonding at the plug/wall interface. Yield problemsassociated with this technique do not make it a viable commercialoption.

Tubes have also been produced using an extruded closed end technique.This technique utilizes a removable die cap that defines the outerhemispherical radius of the close end. With this die cap in place,material is extruded until the closed end is formed. The extrusionpressure is then reduced to zero and the die cap is removed. Extrusionis started again until the required tube length is obtained. Althoughthis technique is an improvement over past methods with respect toclosed end homogeneity, it is a start/stop extrusion process which takesa substantial amount of time to perform. In high volume extrusionmanufacturing operations, the homogeneity and reproducibility of theextruded product is enhanced by continuous flow as opposed to repeatedapplication and removal of the extrusion load. Closed ends fabricatedusing this multi-step extrusion process method are not net shape andrequire post-sintering machining. Additionally, this technique cannot beused to produce closed end ribbed tubular air electrodes.

SUMMARY OF THE INVENTION

The present invention provides a method in which a closed end ceramicSOFC tube is formed by joining a cap to a hollow ceramic tube. Thecross-sectional geometry of the ceramic tube may be round, square or anyother desired geometric configuration. The ceramic tube may optionallyinclude at least one integral rib. The cap may be flat, hemispherical orany other suitable configuration. The cap and the hollow tube arepreferably joined by means of a compound joint, such as a rabbet jointor the like. The closed end tube may comprise an air electrode suitablefor use in fuel cells. As used herein, the term “fuel cell” includesSOFCs, oxygen/hydrogen generator type solid oxide electrolyteelectrochemical cells, solid oxide electrolyte cells, oxygen sensors andthe like.

An object of the present invention is to provide an improved method ofmaking a closed end ceramic fuel cell tube.

Another object of the present invention is to provide a method of makinga closed end ceramic fuel cell tube. The method includes the steps ofproviding an unfired ceramic fuel cell tube, bonding an unfired end capto an end of the unfired ceramic fuel cell tube to form a compoundjoint, and firing the ceramic fuel cell tube and end cap to form theclosed end ceramic fuel cell tube.

These and other objects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic sectional view of a solid oxide fuelcell showing an air flow path during operation of the cell.

FIGS. 2a-2 c are partially schematic side sectional views showing aprocess for forming a closed end fuel cell tube in accordance with anembodiment of the present invention.

FIG. 3 is a side sectional view of a ribbed cylindrical air electrodeincluding an end cap made in accordance with an embodiment of thepresent invention.

FIG. 4 is a cross-sectional view taken through section 4—4 of FIG. 3.

FIG. 5 is a side sectional view of a flattened rib cell including an endcap made in accordance with another embodiment of the present invention.

FIG. 6 is a cross-sectional view taken through section 6—6 of FIG. 5.

FIGS. 7a-7 f are partially schematic side sectional views of ceramicfuel cell tubes illustrating different embodiments of flat end caps inaccordance with the present invention.

FIGS. 8a-8 c are partially schematic side sectional views of ceramicfuel cell tubes illustrating different embodiments of hemispherical endcaps in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A closed-end SOFC tube 10 is shown schematically in FIG. 1. Air A isintroduced into the cell 10 by a ceramic injector tube 12 that deliversair to the closed end 14 of the tube. The closed end 14 of the cell 10provides an air return, allowing the air A to flow through the entirelength of the cell 10 from the closed end 14 to the open end 16. Theintegral air return manifold comprising the air injector tube 12 and theclosed end 14 of the cell 10 coupled with a controlled leakage seal (notshown) at the open end 16 of the cell provides a conventional seal-lessdesign that does not require absolute or high integrity seals betweenfuel and air, and which accommodates differential thermal expansionbetween cells.

The method of the present invention involves bonding an unfired greenbody cap to an unfired green body tube. This process is illustratedschematically in FIGS. 2a-2 c. First, hollow tubes 20 are extruded anddried using any suitable conventional technique. For example, for an airelectrode of a SOFC, the ceramic fuel cell powder may compriseLa_(1−x)(M1)_(x)Mn_(1−y)(M2)_(y)O₃, where x ranges from 0 to 0.5; M1consists of calcium, strontium, yttrium, cerium, other appropriatedopants, or combinations thereof; y ranges from 0 to 0.5; and M2consists of nickel, chromium, zinc, cobalt, other appropriate dopants,or combinations thereof. The solvent may comprise water, propanol, butylacetate, or butoxyethanol, with water being preferred for manyapplications. In addition to the ceramic fuel cell powder and solvent,the mixture may include organic binders such as methylcellulose,hydroxypropyl methylcellulose, polyvinyl alcohol, polyvinyl butyralresin, or acrylic polymer, and/or may include plasticizers such aspolyethylene glycol, butylbenzyl phthalate, or polymeric fatty acids.

The fuel cell tube 20 may be formed by any suitable method, preferablyextrusion. For example, a paste may be made by combining an appropriatemixture of the compounds given above and mixing them under conditions ofhigh shear. An appropriate paste composition could include 70 to 90weight percent air electrode powder, 5 to 20 weight percent water, 1 to15 weight percent hydroxypropyl methylcellulose, and 0.1 to 5 weightpercent polyethylene glycol. The tube may then be extruded by forcingthe paste through a die at elevated pressure (e.g., 800 to 5,000 psi).The shape of the die determines the cross-sectional geometry of theextruded tubes.

The end cap 22 is made in a separate process, preferably by eitherextrusion or die pressing. In the case of extrusion, flat ribbons arepreferably extruded using the same paste formulation as the tube toproduce a thickness that is equivalent to that of the wall of theunfired tube. From this ribbon, disk-shaped caps are cut. Alternately, adry blend of ceramic powder and binder can be uniaxially pressed toyield either a disk-shaped cap or a hemispherical cap having aconfiguration which forms a complex joint when assembled with the tube,as more fully described below. In this case, a dry formulationconsisting of 80 to 98 weight percent air electrode powder, 0.5 to 10weight percent hydroxypropyl methylcellulose, and 0.01 to 2 weightpercent polyethylene glycol is preferred. The resulting mixture isplaced in an appropriately sized and shaped die upon which uniaxialpressure in the range of 200 to 10,000 psi is applied to form the endcap.

As shown in FIGS. 2a-2 c, the end cap 22 is joined to the hollow tube 20to form a compound joint. In the case of aqueous extrusion pastesystems, a diluted paste formulation or slurry 24 is used to achievethis bond. The slurry 24, shown schematically in FIGS. 2a and 2 b, isapplied to the end of the tube 20. The cap 22 is placed over the slurry24 and this assembly is allowed to dry to form a compound joint as shownin FIG. 2c. Drying is preferably performed in a vertical orientationsuch that the weight of the tube 20 aids in the bond. This sequence ofsteps may be automated. After the tube 20 and end cap 22 assembly isdried, it is fired using conventional sintering parameters. For example,sintering temperatures of from about 1,350 to about 1,650° C. andsintering times of from about 0.5 to about 10 hours may be used.

In one embodiment of the present invention, the method may be used tomake ribbed air electrodes for use in high power density solid oxidefuel cells. The presence of ribs in the air electrode tubes preventsmost standard plugging methods from being used in these cell types.However, the present method allows closed end ribbed air electrodes andfuel cells to be fabricated. Examples are shown in FIGS. 3-6.

FIGS. 3 and 4 show views of a ribbed cylindrical air electrode tube 30.The air electrode tube 30 has a circular cross-section and an internalrib 32 which bisects the tube. An opening 33 is provided at the bottomof the rib 32 in order to allow gas to flow from one interior section ofthe air electrode 30 to the other interior section. An end cap 34 isconnected to the bottom of the air electrode tube 30 to form a compoundjoint in accordance with the present invention. As shown in FIG. 3, thepresent process produces a compound joint in which the cap 34 forms ahomogeneous boundary with the air electrode tube 30.

FIGS. 5 and 6 show a closed end flattened ribbed SOFC. In thisembodiment, the air electrode tube 40 has a generally ovular flattenedcross-section. Internal ribs 41, 42 and 43 are provided inside the airelectrode tube 40. Openings 44 and 45 in the ribs 42 and 43 allow air Ato flow through the air electrode tube 40 as shown in FIG. 5. An end cap46 is bonded to the bottom of the air electrode tube 40 to form acompound joint in accordance with the present invention.

Alternate compound joint configurations of the present invention areshown in FIGS. 7a-7 f and 8 a-8 c. In FIGS. 7a-7 f, the air electrodetube 50 is connected to various types of caps 51-56 having generally,flat exterior surfaces and forming compound joints with the tube 50. InFIGS. 8a-8 c, the air electrode tube 60 is connected to various types ofend caps 61-63 having generally hemispherical shapes and formingcompound joints with the tube 60. In accordance with the presentinvention, the use of compounds joints, such as those shown in FIGS.7a-7 f and 8 a-8 c, increase the bond area between the cap and the tubewall, thereby providing an improved seal.

The present method may use aqueous extrusion paste systems based onhydroxypropyl methylcellulose ether. However, the process may also becompatible with other aqueous systems or non-aqueous systems thatutilize thermoplastic materials. The bonding of the end cap to the tubein the case of a thermoplastic system would require localizedapplication of heat, rather than a slurry.

An air electrode having an end cap in accordance with the presentinvention may be fabricated into a complete SOFC by conventionalmethods. For example, electrolyte and fuel electrode layers may bedeposited on the air electrode by conventional electrochemical vapordeposition techniques. The resultant cells made with the closed-endtechnique of the present invention are substantially leak-tight.

The present invention has several advantages over the prior art. The useof a compound joint between the cap and the fuel cell tube provides arelatively large bond area between the components which reduces the riskof gas leaks. The method does not require elaborate dies or fixturing,and no special drying equipment is required. When the end cap is formedfrom the same extrusion mix as the tube wall, and both are in a fullydried green state, there are substantially no differential shrinkageproblems that could give rise to a poor cap/wall bond. Additionally, theporosity of the resultant fired closed end is substantially identical tothe tube wall adjacent to the closed-end. The method of the presentinvention is particularly suited for forming closed end ribbed airelectrode tubes.

The present method also allows for the continuous extrusion of tubes.This is in contrast with the conventional extruded plug technique, whichis a start/stop extrusion process. In accordance with the presentinvention, extruded product homogeneity and reproducibility is enhancedby continuous flow rather than the repeated application and removal ofthe extrusion load. The present process also allows for very rapidextrusion of tubes, and is compatible with large scale tubemanufacturing operations. The present end cap technique is well suitedto such high volume processing. Furthermore, with the present method, nogrinding or machining of the sintered air electrode tube is required.This is in contrast with conventional pressed plug, cast plug andextruded closed end techniques.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

What is claimed is:
 1. A method of making a closed end ceramic fuel celltube comprising: providing an extruded, dried and unfired ceramic fuelcell tube; bonding an extruded, dried and unfired end cap to an end ofthe unfired ceramic fuel cell tube to form a compound joint; and firingthe ceramic fuel cell tube and end cap to form a sintered closed endceramic fuel cell tube comprising a sintered ceramic fuel cell tube anda sintered end cap having substantially the same porosities andthickness.
 2. The method of claim 1, wherein the compound jointcomprises at least one recessed annular ring provided in at least one ofthe unfired ceramic fuel cell tube and the unfired end cap.
 3. Themethod of claim 1, wherein the compound joint comprises an annularrecess provided in the unfired end cap.
 4. The method of claim 1,wherein the compound joint comprises an annular recess provided in theunfired ceramic fuel cell tube.
 5. The method of claim 1, wherein thecompound joint comprises conforming annular recesses in the unfiredceramic fuel cell tube and in the unfired end cap.
 6. The method ofclaim 1, wherein the end cap is substantially flat.
 7. The method ofclaim 1, wherein the unfired ceramic fuel cell tube and unfired end capare of substantially the same composition.
 8. The method of claim 1,wherein the unfired end cap is bonded to the unfired ceramic fuel celltube by providing a slurry between the unfired end cap and the unfiredceramic fuel cell tube, and drying the slurry.
 9. The method of claim 1,wherein the unfired ceramic fuel cell tube has a substantially circularcross section.
 10. The method of claim 1, wherein the unfired ceramicfuel cell tube comprises at least one internal rib.
 11. The method ofclaim 1, wherein the sintered closed end ceramic fuel cell tubecomprises an air electrode of a solid oxide fuel cell.
 12. The method ofclaim 11, wherein the air electrode comprises doped-lanthanum manganite.